1. Field of the Invention
The present invention relates to digital radio communication systems generally, and, more particularly, to time division multiplex digital radio communication systems and to process of operating time division multiples digital radio communication systems.
2. Description of the Related Art
In a time division duplex (TDD) digital radio communication system such as the cordless telephone second generation (i.e., CT-2), a transmission carrier frequency is identical to a reception carrier frequency. The time division multiples digital radio communication system alternates transmission and reception at intervals of specific times. Hence, the transmission intermediate frequency is also identical to a reception intermediate frequency, so that during reception, a signal source for the transmission intermediate frequency may cause interference with the reception intermediate frequency. In order to eliminate this interference, a conventional time division duplex digital radio communication system disables (or turns to an operationally off state) an radio frequency receiver during the transmission mode and disables the radio frequency transmitter during the reception mode.
In contemporary designs of time division duplex digital radio communication system, a first local oscillation frequency is used for both the transmission and reception, to generate the transmission intermediate frequency. The local oscillator however, generates harmonic components with substantial amplitudes. Although the Time Division Multiple Access Time Division Duplex Type Transmitter-Receiver of Y. Hirose, U.S. Pat. No. 5,598,405 advocates placing the phase locked loop in an off state at all times (including the transmission time slot and the reception time slot) other than time slots just before the transmission time slot and the reception time slot, I have found that it is not feasible to enable and disable the local oscillator during the transmission and reception modes principally because of the gross disparity between the guard times between the transmission and the reception, and the switching stabilization (or settling) time of the local oscillator, and the phase locked loop oscillator--the switching stabilization time for the local oscillator is over ten times the guard time of the digital radio communication system, so that it is not feasible to turn the local oscillator on and off in correspondence with the transmission and reception modes.
Generally, it is desirable to maintain a possible reception sensitivity between -100 dBm and -108 dBm in a time division duplexer digital radio communication system by the expedient of maintaining interference signals between -113 dBm and -121 dBm. It is very difficult to restrict the third harmonic signals contained in the output of the local oscillator below -30 dBm. Thus, it is necessary to maintain an isolation degree of over about 90 dB, which is, however, unattainable due to the circuitry and spatial limitations. In addition, in conventional digital radio communication systems and applied to a voltage controlled oscillator. Since the first local oscillation frequency is used in common for transmission and reception, it must be activated in both the transmission mode and the reception mode because the reception frequency should be identical to the transmission frequency as long as the frequency error is not generated. A phase locked loop for generating the first local oscillation frequency is always closed so that the voltage controlled oscillator is controlled by a phase locked loop frequency synthesizer. When the input transmission data is applied to the voltage controlled oscillator, the loop bandwidth of the phase locked loop is limited in both the high frequency band and the low frequency band. If the loop bandwidth of the frequency synthesizer increases over about 10 KHz, distortion due to the noise and pseudo signal in the loop also increases. Further, if distortion is decreased below several hundreds of Hertz, the stability of the loop is adversely reduced, so that the frequency and phase locked status will not be attained and the drift noise increased. Moreover, if a single point modulation is performed onto the voltage controlled oscillator in the phase locked loop, the loop response to the modulation signal exhibits a highpass filter feature which attenuates the low frequency signals. Accordingly, in order to increase the bandwidth responsive to the modulation signal, the design of contemporary systems suggests that the loop bandwidth should be reduced to the minimum value, a contradictory requirement. As the result, the modulated transmission signal may be attenuated and distorted at the low frequency band. Conventional isolation techniques such as that of D. E. Fague in U.S. Pat. No. 5,515,364 for a Radio Frequency Telecommunications Transceiver, and that found in U.S. Pat. No. 5,553,317 for a Quadrature Modulator For TDMA/TDD Radio Communication Apparatus of T. Hara, have been limited to the design of the circuit itself, while other designs have focused upon the printed circuit board, and the mechanism of the circuit, with an undesirably increase the size, power consumption, and material costs of the system.